Arachnoids are one of the more remarkable features found on
Venus. They are seen on radar-dark plains in this Magellan image mosaic
of the Fortuna region. As the name suggests,
arachnoids are circular to ovoid features with concentric rings and
a complex network of fractures extending outward.
The arachnoids range in size from approximately 50 kilometers (29.9
miles) to 230 kilometers (137.7 miles) in diameter. Arachnoids are
similar in form but generally smaller than coronae
(circular volcanic structures surrounded by a set of ridges and
grooves as well as radial lines). One theory concerning their
origin is that they are a precursor to coronae formation. The
radar-bright lines extending for many kilometers might have resulted
from an upwelling of magma from the interior of the planet
which pushed up the surface to form "cracks." Radar-bright lava
flows are present in the 1st and 3rd image, also
indicative of volcanic activity in this area. Some of the
fractures cut across these flows, indicating that the flows
occurred before the fractures appeared. Such relations between
different structures provide good evidence for relative
age dating of events.
At present, arachnoids are found only on Venus and can now be more
closely studied with the high resolution (120 meter/0.07 mile)
radar imagery from Magellan.